A male Warden gets a choice in it when it comes to Zevran, though. Male mage Warden wouldn't have gotten a choice if that had been part of the game. That's the point. And of course he doesn't bring up feelings for a male Warden in DAI, they decided against making him a bisexual LI. If they had gone through with it, perhaps he would have. We'll never know since it's all just theoretical at this point. However, unless someone specifically says they're not attracted to the same sex, all we can do is assume they're straight. Which is fine, but it's not retconning if our assumptions prove to be false since they were, after all, just assumptions.
While I don't think bi DA:I Cullen would have been a "retcon" in the sense of "an m/m romance could not possibly be written in such a way that it would have been consistent with his earlier character" (only in the sense of "authorial intent changed"), I think DA:I Cullen saying anything about the male Warden would definitely have been one. His attraction to the female Warden was revealed partially by involuntary stuttering, partially by torture from blood mages and demons who learned of his attraction by reading his mind, and only secondarily by his actually consciously saying anything remotely flirty to her.
I'm not saying bisexuals should have to wear a forehead sticker, or that they have to like men and women equally, or that characters like Cassandra and Varric in DA2 who had been in individual opposite-sex relationships were necessarily straight until they identified as such in DA:I. But I see a lot of the idea that a biphobic / heteronormative viewpoint is the only reason to have considered DA:O Cullen straight, and I just can't agree with that. The only determining factor in his attraction to the warden was gender. If he didn't realize he was bi until he was older or something, fine, but I find the "well, maybe he knew he was bi then and just didn't like a single one of your eleventy billion possible male Wardens, even though he liked all eleventy billion female Wardens" argument really unsatisfying, and the "maybe he just successfully hid his attraction to your m!Amell/Surana through weeks of blood magic torture" argument dubious.
And to be clear, I don't think mine is the only valid viewpoint. Just that it is a valid one.
I know what you mean, but generally Cullen at that point is neither straight nor gay, nor bi. He exist to provide a joke.
Or, you know, to crank up the pathos for mages later in the game, like Tamlen and Gorim do for Dalish Elves and Dwarven Nobles. Or to efficiently illustrate several aspects of life in the Circles. I thought his little mini-romance was very elegant in terms of information conveyed per word. The "joke" part was one optional line.